1. Field of the Invention
The present invention relates to a damping force control system for an automotive vehicle for controlling a semiactive damper mechanism provided between an unsprung mass and a sprung mass of the vehicle to dampen vertical vibration of the sprung mass. More particularly, the present invention relates to a damping force control system capable of switching over the semiactive damper mechanism from a low damping state to a high camping state in response to an increase of vertical vibration of the sprung mass.
2. Description of the Prior Art
Disclosed in Japanese Patent Laid-open Publication No. 3(1991)-276806 is a control system for a semiactive damper mechanism wherein vertical velocity of a sprung mass of an automotive vehicle and relative velocity between an unsprung mass of the vehicle and the sprung mass are detected to control a damping force of the damper mechanism against the vertical vibration of the sprung mass. In such a conventional control system, the damper mechanism is switched over to control the damping force when the relative velocity between the sprung mass and the unsprung mass is approximately zero thereby to decrease an impact applied thereto when it has been switched over in a condition where the relative velocity between the sprung mass and the unsprung mass is other than zero. This is effective to reduce undesirable shocks and noises caused by switchover of the damping force. In the conventional control system, it is, however, required to provide a sensor for detecting relative velocity between the sprung mass and the unsprung mass. Since the sensor is, in general, mounted on the vehicle body at a position located near the surface of a travel road and exposed to the outside air, there will occur a problem in durability of the sensor.